Quasi-satellite
A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet where the object stays close to that planet over many orbital periods. A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity (usually greater), as shown in the diagram on the right. When viewed from the perspective of the planet, the quasi-satellite will appear to travel in an oblong retrograde loop around the planet. In contrast to true satellites, quasi-satellite orbits lie outside the planet's Hill sphere, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighbourhood, then possibly later move back to a quasi-satellite orbit, etc. Other types of orbit in a 1:1 resonance with the planet include horseshoe orbits and tadpole orbits around the Lagrangian points, but objects in these orbits do not stay near the planet's longitude over many revolutions about the star. Objects in horseshoe orbits are known to sometimes periodically transfer to a relatively short-lived quasi-satellite orbit, and are sometimes confused with them. An example of such an object is . Examples ;Earth As of 2014, Earth has several known quasi-satellites: , , and . 3753 Cruithne, , and are minor planets in a horseshoe orbit that can transition into a quasi-satellite orbit. was in a quasi-satellite orbit from 1996 to 2006. ;Venus Venus has a quasi-satellite, . This asteroid is also a Mercury- and Earth-crosser; it seems to have been a "companion" to Venus for the last 7000 years or so only, and is destined to be ejected from this orbital arrangement about 500 years from now. ;Neptune is a temporary quasi-satellite of Neptune. The object has been a quasi-satellite of Neptune for about 12,500 years and it will remain in that dynamical state for another 12,500 years. ;Other planets Based on simulations it is believed that Uranus and Neptune could potentially hold quasi-satellites for the age of the Solar System (about 4.5 billion years), but a quasi-satellite's orbit would remain stable for only 10 million years near Jupiter and 100,000 years near Saturn. Jupiter and Saturn are known to have quasi-satellites. ;Artificial In early 1989, the Soviet Phobos 2 spacecraft was injected into a quasi-satellite orbit around the Martian moon Phobos, with a mean orbital radius of about from Phobos. According to computations, it could have then stayed trapped in the vicinity of Phobos for many months. The spacecraft was lost due to a malfunction of the on-board control system. ;Accidental Some objects are known to be accidental quasi-satellites, which means that they are not forced into the configuration by the gravitational influence of the body of which they are quasi-satellites. The minor planets Ceres, Vesta, and Pluto are known to have accidental quasi-satellites. In the case of Pluto, the known accidental quasi-satellite, , is, like Pluto, a plutino, and is forced into this configuration by the gravitational influence of Neptune. This dynamical behavior is recurrent, the object becomes a quasi-satellite of Pluto every 2 Myr and remains in that configuration for nearly 350,000 years. See also * Natural satellite References External links *Quasi-satellite Information Page *Earth's New Travelling Companion: Quasi-Satellite Discovered *Astronomy.com: A new "moon" for Earth *Discovery of the first quasi-satellite of Venus – University of Turku news release (August 17, 2004) Category:Moons Category:Co-orbital minor planets